Episode: "Another Mister Sloane" (Season 4, Episode 16)
Alias routinely displayed some of the best action choreography to ever be seen on the tube, then and now. But the odd spy series also had a supernatural-mystic lean that threatened to overtake the show during some frustrating mid-seasons.
It also allowed for sloppy CGI to slip in at times and taint the show's overall legacy. One example: the Mueller device, a huge red ball that was first seen in the pilot. It's seen several instances during the Alias' run, but, curiously, the first season's episodes that offer a glimpse at the round weapon are more or less solid. It's this late season four mission that offers up the first truly laughable look at the contraption. In turn, it inspires doubt in regards to the entire story arc.
Coincidentally, season four was the show's weakest one. Go figure.